AMjaJnOT^'i874.BM'}      Examination  of  Blood  Stains.  31 
1.  Spectrum  Analysis. — Coloring  matters  have  the  power  of  absorb- 
ing certain  colored  rays  of  white  light — the  same  always  for  the 
same  substance.  This  is  the  principle  on  which  spectroscopic  exam- 
ination is  based.  If  into  an  analysing  tube  filled  with  water  a  few 
drops  of  a  solution  of  haemoglobin  be  introduced  till  it  has  the  color  of 
peach-blossoms,  the  luminous  rays  of  the  spectrum  passing  through  this 
fluid  present  two  bands  of  absorption  between  the  lines  D  and  E  of 
Frauenhofer  in  the  yellow  and  the  green.  The  same  fact  would  be  ob- 
served if  a  few  drops  of  blood  were  substituted  for  haemoglobin  in  the 
analysis.  In  a  case  of  doubt  the  haemoglobin  of  the  blood  could  be 
reduced  by  adding  to  the  latter  areducing  body.  Destroyed  haemoglobin 
has  a  different  spectrum  from  oxygenated  haemoglobin ;  a  single  ab- 
sorption band  as  large  as  the  two  former  bands  united,  and  a  little  to 
the  left  of  Frauenhofers  line  D. 
2.  In  blood  in  a  state  of  decomposition,  or  which  has  been  treated 
by  acids  or  caustic  alkalies,  haemoglobin  is  changed  into  a  new  sub- 
stance ;  haematin  is  formed,  which,  combined  with  hydrochloric  acid, 
gives  characteristic  crystals.  In  order  to  obtain  them,  we  must  pro- 
ceed thus.  A  small  fragment  of  dried  blood  is  placed  on  a  glass  slide  ; 
it  is  dissolved  in  a  drop  of  water,  and  a  minute  portion  of  sea-salt  is 
added.  It  is  covered  with  a  thin  slide,  and  pure  acetic  acid  is  made 
to  pass  between  the  two  slides,  and  it  is  heated  over  a  spirit-lamp,  to 
boiling  point.  Acetic  acid  is  again  added,  and  it  is  heated  afresh, 
and  this  is  repeated  till  the  crystals  are  obtained.  They  are  rhom- 
boidal,  of  a  dirty  brown  color,  quite  characteristic,  and  require  to  be 
seen  with  a  magnifying  power  of  three  hundred  or  four  hundred  diam- 
eters. With  the  smallest  quantity  of  blood  these  two  reactions  can 
always  be  produced — the  spectrum  examination  and  the  crystals  of 
hydrochlorate  of  haematin  ;  and  they  are  so  certain,  that  the  existence 
of  one  alone  enables  one  to  affirm  the  presence  of  blood. 
3.  The  third  process,  though  not  so  exact  as  the  preceding,  ought 
nevertheless  not  to  be  neglected.  If  to  a  very  small  quantity  of  blood 
dissolved  in  a  little  water  be  added  a  few  drops  of  tincture  of  guiai- 
cum  and  of  binoxide  of  hydrogen,  a  persistent  blue  color  is  immediate- 
ly produced ;  but  this  very  sensitive  reaction  can  be  obtained  with 
other  organic  matter,  nasal  mucus,  saliva,  &c;  it  therefore  only  gives 
a  probability.  We  must  proceed  in  the  following  manner.  A  tinc- 
ture of  guiaicum  is  prepared  with  alcohol  at  83  degrees,  and  guiaicum 
resin ;  a  mixture  of  sulphuric  ether  and  binoxide  of  hydrogen  is  also 
